Older builds of our game had some problems with the inventory system. I feel confident in the fact that we've got those errors sorted out now, but unfortunately those old errors can get stuck in the persistent save data if you are loading from an older save. Not all older saves have this problem, and I'm not sure exactly sure what triggered the error in a small percentage of playthroughs, but once the error is there, it's hard to get rid of. Once you've got this error, the only sure fire way I've found to fix it is to nuke all your save data. You do this by deleting all the files in your save folder in the folder named "game" and then you need to delete the AFV folder in your AppData folder (C:\Users\YourProfileName\AppData\Roaming\RenPy\AFamilyVenture)
I know this solution sucks, and I'm sorry because you will lose all of your progress, but so far it's done the trick for everyone I've come across with this problem. You can ask for a save from someone else, but you've got to be weary since if their save has that same error it will corrupt your persistent save data once again.